Voyager sets sights on Milky Way

An international group of researchers has reported the first ever detection of a particular atomic hydrogen line emission from the Milky Way, using instruments on board the Voyager spacecraft. This emission, has already been seen from much more distant objects and is used to find star-forming galaxies. It is also used to probe the epoch of reionization – the formation of the first stars post the "dark ages" of the universe – and so is of considerable significance to astronomers.

Astronomers routinely look at astronomical objects and phenomena that are many millions of light-years away – currently, the furthest object observed is at a distance of 13.14 billion light-years. But surprisingly, there are phenomena that occur within the bounds of our parent galaxy – the Milky Way – that have not been seen or studied. An example of this is the "Lyman-alpha (Lyα) emission" (121.6 nm) that is generated when there is an electron transition between the first and second energy levels of hydrogen. The galactic Lyα emission is an essential marker of the young-star-formation rate in the Milky Way, the ionization environment in which the atmospheres of young planets evolve, and the amount of shocked gas in the interstellar medium.

Star birth marker

Although Doppler-shifted Lyα lines emission has been seen from other galaxies, it has been undetectable for the Milky Way as a result of very bright local sources that drown out the galactic Lyα radiation, in a similar manner to which the glare of bright lights from a city blocks the light from all but the brightest of stars. This local brightness is mainly attributed to the "H glow" – solar Lyα photons scattered by neutral hydrogen gas in the solar system. Like getting away from a big city to see a clear sky, astronomers are using data from the two Voyager spacecraft – both of which have now reached the heliosheath at the very edge of the solar system and are beyond the worse of the H glow.

Using the recently acquired data from the Voyager spacecraft, launched by NASA in 1977, Rosine Lallement from the Observatoire de Paris run by the French research council (CNRS), and colleagues in the US and Russia are the first to study this galactic emission and confirm that most of it originates in the regions where hot young stars are being formed. As both spacecraft are moving out of the solar system, they can perceive the much fainter radiation that comes from the galaxy. "For us, it is like beginning to see small candles within a brightly lit room," explains Lallement. The team has been busy "disentangling the two [local and distant] signals" that have two different consequences.

Glowing Milky Way

In the distant case, the astronomers study the amount of ultraviolet Lyα radiation emitted by a galaxy because it corresponds to the rate at which stars are being born within that galaxy – that is, the star formation rate (SFR) of the galaxy. Lallement explains that one of the major goals for astronomers is to pinpoint when stars first appeared in the young universe, and so detecting the Lyα emissions from the most distant galaxies and correctly interpreting the signal is essential. "However, the correspondence between Lyα and the SFR is not an easily derived due to the complex manner in which the radiation propagates through the distant galaxy. A single Lyα photon can be absorbed and scattered by the numerous neutral hydrogen clouds present within galaxies, and hence its history is essentially lost due to its complex "random walk" from its origin to its escape from the ionized regions of a distant galaxy."

She goes on to say that the star, gas and dust distributions are not known in those galaxies that are extremely faint and so accurately observing and deciphering the Lyα galactic signal to test and calibrate Lyα photon propagation models for distant galaxies was impossible. "In the case of the Milky Way we have for the first time the Lyα signal and all the necessary information, thus models can be tested," says Lallement. The Lyα emission can trace the SFR in much more distant galaxies, with redshifts from z = 2 to z = 6.

Closer to home

The study of the other signal – the local signal – is important to understand the heliosphere – the bubble formed due to the solar wind that contains our entire solar system and marks the extent of the Sun's environment, including the boundary between the Sun and the ambient galactic interstellar medium. Both Voyager spacecraft are currently crossing the boundary region, moving into the galactic gas. "The H glow due to penetrating neutral hydrogen atoms from interstellar space is part of the whole structure. Understanding how this local H glow evolves with the distance to the Sun and confirming the best models of the glow brings information that is complementary to in situ data", says Lallement, explaining that although the data from the Voyager spacecraft shows a preliminary distribution of the emissions; precise maps will have to wait a dedicated mission. NASA's New Horizons spacecraft, on its way to Pluto, has an ultraviolet-imaging spectrometer that could observe galactic Lyα emission in a more systematic way.

Unfortunately, power on board the Voyager spacecraft decreases as they move further and further away from the Sun. Indeed, no data will be received beyond 2020–2025. To save power, the UV instruments on board are no longer capable of pointing towards a certain source; data are still recorded but in a fixed direction. Hopefully, they will still generate new and useful information about the galactic Lyα emissions as well as the interstellar gas until then.

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9 comments

An other way?

The signals of Ly-alpha from the Milky Way sent by the voyager sapcecraft from beyond the rim of the helioshpere should be relatively pure to reperesent the rate of formation of stars in the it. However, the problem remains unsolved for the distant galaxies due to the pollution introduced in the intensity of this Ly-alpha line due to absorption and scattering by the intervening gas and dust. Is there not an other way of determinig tha rate of star formation?

Galatic radiation and CMB

This story reminds us of the discovery of C.M.B. by Arno Penzias and Robert Wilson in 1965. Can any one compare Galactic radiation with C.M.B., for enlightening the public? The comparison would be useful because the year 2015 will be celebrated as the International Year of Light and 50 years of the discovery of C.M.B. is one reason for the celebration.

cmb and ly-alpha radiations are quite different

The source of the cmb radiation is the decouplig of the EM radiation from matter after about 480000 years after the BB. The energy of this radation decreased to the present value of about 2.7°K everywhere in the universe after about 13 billion years due to its expansion. However, the origin of the Ly-alpha radiation is from the excitation of hydrogen atoms from local sources of star formation in a galaxy. It is emitted continuously and is detected rather soon after its emission..quote=Dileep Sathe;15769]This story reminds us of the discovery of C.M.B. by Arno Penzias and Robert Wilson in 1965. Can any one compare Galactic radiation with C.M.B., for enlightening the public? The comparison would be useful because the year 2015 will be celebrated as the International Year of Light and 50 years of the discovery of C.M.B. is one reason for the celebration.

Good Q

This story reminds us of the discovery of C.M.B. by Arno Penzias and Robert Wilson in 1965. Can any one compare Galactic radiation with C.M.B., for enlightening the public? The comparison would be useful because the year 2015 will be celebrated as the International Year of Light and 50 years of the discovery of C.M.B. is one reason for the celebration.

A good Q Prof.- I understand the Voyager is at ~117AU from our-Sun- if that's true (let NASA confirm on that), I estimate CMB at that point would be close to 2.5K... Let NASA or the scientist or the writer of this article confirm the actual recorded data by Voyager... I hope NASA would disclose it, soon...

What is going on ?

S. M. Krimigis finds (from the data of NASA Voyager 1 & 2, see Nature 474, 359-61, June 16, 2011), Zero outward flow velocity of plasma at a distance ~116 AU from the Sun. I read the other articles published in Science on Voyager 1 & 2, including this one by R.Lallement...(which is silent as to the present position or location of V1 or V2 from our-Sun; also, nothing about temperature other than low brightness at 40R- whatever that meant with respect to our-Sun's brightness?). But, the other articles in Science published in Nov. 2011 gives the position of V1 at 94AU from Sun(in 2004) and V2 at 84AU (in 2007)... Why did not the Science authors provide the location of V1 & V2 as of 2011, like Krimigis-article in Nature of June 2011 who came up with 116AU? What is going on- where exactly V1 & V2 are at the present time? What is correct Science at 94AU or Nature at 116AU? And, how did they determine that location distance? I seriously suspect the correctness in the concept of these authors which advocates that V1 & V2 have "now reached the heliosheath at the very edge of the solar system and are beyond the worse of the H glow"- they have no idea what they are talking about. I am also forwarding this comment to Science & Nature editors to clarify on the finding...

Yes, I got some answers

I spoke to a Professor at Caltech and I am told V1 is at 119 AU and V2 is at 97 AU- they are moving away from Sun to a point of no-return at angle of 35 degrees to the Sun-Earth line (i.e ~70 degree angle between V1-V2). And, there is no sensor on V1 or V2 to record the outside temperature...I made a suggestion that if V2 (being at 97 AU) can be re-routed to at ~60 degrees (instead of 35), it is possible it could encounter a 'dark-Object' within 10-20 AU... I hope NASA or another authority who might have control on this V2 (which is not going to do any useful purpose hereafter...) would consider re-routing it to discover an extra-ordinary Object... that controls our-Sun and the planets...

Which way V1 or V2 to be re-routed?

I spoke to a Professor at Caltech and I am told V1 is at 119 AU and V2 is at 97 AU- they are moving away from Sun to a point of no-return at angle of 35 degrees to the Sun-Earth line (i.e ~70 degree angle between V1-V2). And, there is no sensor on V1 or V2 to record the outside temperature...I made a suggestion that if V2 (being at 97 AU) can be re-routed to at ~60 degrees (instead of 35), it is possible it could encounter a 'dark-Object' within 10-20 AU... I hope NASA or another authority who might have control on this V2 (which is not going to do any useful purpose hereafter...) would consider re-routing it to discover an extra-ordinary Object... that controls our-Sun and the planets...

First, I need to understand clearly which way V1 & V2 moving? Are they in front or behind the Sun? And, where are they located with reference to Sun's N-S poles and/or with reference to Sun's equator plane? Second, then- I can suggest the re-routing to get to the 'dark-Object... Let us wait and see if NASA picks up on that thru Caltech or JPL?